Hydrocarbon separation by azeotropic distillation with trioxane



Patented Dec. 16, 1947.

UNITED STATES PATENT OFFICE HYDROCARBON SEPARATION BY- AZEO- TROPICDISTILLATION WITH TRIOXANE No Drawing. Application November 1, 1943,

Serial No. 508,639

7 Claims. 1

This invention relates to the preparation of pure hydrocarbons frompetroleum, these pure hydrocarbons being contained in a fraction ofpetroleum hydrocarbons whose components have small differences inboiling points, which renders them inseparable by ordinary fractionaldistillation. This is a continuation-in-part of my copending applicationSerial No. 412,814, filed September 29, 1941.

Another object of the invention is to prepare from a given fraction ofpetroleum, such as gasoline, kerosene, or a narrow boiling rangehydrocarbon fraction prepared from such materials, these fractionsconsisting of a mixture of paraffinic, isoparaflinic, naphthenic,olefinic and aromatic hydrocarbons, a fraction that is essentiallyparaffinic or isoparafiinic or naphthenic or olefinic or aromatic bydistilling such fractions of petroleum in the presence of trioxane.

A particular object of my invention is to separate aromatic hydrocarbonsfrom non-aromatic hydrocarbons by distilling the complex hydrocarbonfraction in the presence of trioxane.

The invention comprises adding to such petroleum fractions from which itis desired to segregate a specific hydrocarbon or hydrocarbon fraction,trioxane having a preferential afllnity for one or more componentscontained in the fractions, thus causing a disturbance of the vaporpressure equilibrium that formerly existed in the fraction, in suchmanner that the partial vapor pressure or iugacity of at least onecomponent in the fraction is changed suificiently to permit itsseparation by controlled fractional distillation. This type offractional distillation will be referred. to hereinafter as azeotropicdistillation and the trioxane will be referred to as azeotrope former.

According to my invention, the separation of a specific hydrocarbon orhydrocarbon fraction from a mixture of hydrocarbons is accomplished byazeotropic distillation wherein a azeotrope former consisting oftrioxane is added to the petroleum fraction and the mixture is subjected-to controlled fractional distillation. The addihydrocarbons and theazeotrope former which is more volatile than the aromatic hydrocarbonswhich may or may not contain a portion of the azetrope former. Thefractional distillation of the mixture results in distilling overheadthe naphthene hydrocarbons in admixture with the azeotrope formerleaving the aromatic hydrocarbons as undistilled' bottoms. The sameprocedure may be employed to segregate paraffin and aromatichydrocarbons in which case the paraffin hydrocarbons form a lowerboiling azeotrope with the azeotrope former. Likewise paraflinhydrocarbons may be separated from naphthene hydrocarbons in which casethe naphthene hydrocarbons remain as the undistilled bottoms. While itis preferred to efiect the fractional distillation in such manner thatone of the components in the hydrocarbon fraction remains as anundistilled bottoms, it is also possible to vaporize the mixture ofhydrocarbons completely With the azeotrope former and then by controlledfractionation in a fractionating column effect the condensation of theseparate hydrocarbon components at various points in the fractionatingcolumn from which the various components may be removed.

In such cases where the hydrocarbon fraction contains more than twocomponents of different chemical characteristics, as for example,aromatics, naphthenes, and paraffins, and it is desired to separate oneor more of these components from the other component or components, theseparation may be accomplished by stage fractional distillation toremove first one component and then another component. For example,trioxane may be added to a mixture of aromatics, naphthenes andparaffins having a boiling range of 200 to 240 F. and the mixture thendistilled to remove as overhead fractions, first an azeotrope of theparafiins with trioxane and then an azeotrope of the naphthenes withmore trioxane leaving the aromatics as undistilled bottoms eithercontaining trioxane or not. The

point at which one component, the paraiiins, for example, issubstantially completely distilled from the remaining components may beobserved by a rise in distillation temperature in order to eiiectfurther distillation of the material in the still. Thus, in the aboveexample, the distillation is initially carried out at an overheadtemperature of 206 F. at which temperature the paraffin hydrocarbonstogether with trioxane distill from the remaining hydrocarboncomponents, then when substantially all of the parafiin components havebeen evaporated from the mixture, it will be necessary to raise thedistillation temperatures so that the overhead temperature will beincreased to, for example, 220 F. in order to effect further removal ofhydrocarbon components. This increase in temperature indicates that allof the parafiins were previously distilled from the mixture and that thenext hydrocarbon components, for example the naphthenes, are beingdistilled at the increased temperature. By thus observing andcontrolling the distillation temperature, it is possible to remove thevarious components present in the original feed stock as separatefractions.

While the invention is adapted for the separation of hydrocarbons ofcharacteristics different from each other, I have found that thisprocess is particularly useful for producing toluene or xylene having avery high degree of purity from gasoline fractions produced fromstraight run or synthetic gasoline such as those produced by cracking,polymerization or reforming. The production of substantially puretoluene and xylene is highly important when these compounds are to beused in the manufacture of explosives by nitration because even smallamounts of impurities impair the nitration process and degrade theresulting nitration product.

The above disclosed azeotrope former may be employed in the anhydrousstate in which case the resulting azeotropic distillate will compriseazeotrope former and hydrocarbon material or it may be used with waterin which case the azeotropic distillate will comprise azeotrope former,hydrocarbon material and water. Thus I may use the anhydrous azeotropeformer or I may use the azeotrope former together with water in amountsup to about 0.4 part by weight of water to 1 part of the azeotropeformer.

Trioxane is very eflicient for separating hydrocarbon fractions havingnarrow boiling ranges, preferably not more than 50 F., between thelimits of 150 F. to 330 F. into hydrocarbon components of differentchemical characteristics and is particularly useful for separatingparaffin and/or napthene hydrocarbons having 6 to carbon atoms fromaromatic hydrocarbons having 9 or less carbon atoms, as for example,when separating hexanes, heptanes, octanes, nonanes, decanes and/ornaphthene hydrocarbons having similar numbers of carbon atoms frombenzene, toluene, xylenes and ethyl benzene.

The type of distillation to be used depends somewhat on the quantity ofthe aforementioned azeotrope former used. I may take any proportion ofthe petroleum fraction to the added azeotrope former that I desire,depending on the efficiency of the operation or the purity of theproduct desired, and the technique to be used in the distillation. Theproportion of the azeotrope former may readily be adjusted on an idealpoint, the definition of this point again depending on whether I desirethe portion high in aromaticity to remain as bottoms in the boiler in apractically pure state, i. (3., free from nonaromatic hydrocarbons, orwhether I wish to distill a portion of the non-aromatic hydrocarbons,leaving a portion of the non-aromatic hydrocarbons as bottoms togetherwith aromatic hydrocarbons. Also, the distillation temperature andamount of azeotrope former may be adjusted to effect the distillation ofall of the non-aromatic hydrocarbons together with a portion of thearomatic hydrocarbons. In other words, the efiiciency of separation ofthe aromatic from non-aromatic hydrocarbons is dependent upon the properadlll$tm lll i 95 the amount of azeotrope former used since a smallamount may result in incomplete separation of the non-aromatichydrocarbons while the use of an excess of the azeotrope former togetherwith a relatively higher distillation temperature may cause distillationof a portion of the aromatic hydrocarbons.

In order to separate the azeotrope former from the azeotropicdistillate, it is merely necessary to extract the condensate mixturewith a solvent adapted to extract or dissolve the azeotrope former andsubstantially none of the hydrocarbons. By allowing this mixture tosettle, two distinct layers are formed, an upper layer consisting of thehydrocarbon and a lower layer of azeotrope former dissolved in thesolvent. Solvents useful for the purpose include the nitroparaflins,such as nitromethane, nitroethane, nitropropane, propylene glycol anddiethylene glycol, and even saturated heterocyclic organic compoundshaving different boiling points than the azeotrope former to beseparated from the azeotropic distillate. In some cases, the separationof the azeotrope former from the hydrocarbons may be accomplished bycooling the azeotropic distillat sufficiently, as for example, below F.in order to reject the hydrocarbons from the azeotrope former or tocause the azeotrope former to crystallize and separate from thehydrocarbon mixture. Trioxane is water soluble and is preferablyextracted from the azeotropic distillate with water at an appropriatetemperature to effect the desired result. The azeotrope former may berecovered from the non-aqueous solvent or water by simple distillation,the overhead being either the azeotrope former or the solvent dependingupon the relative boiling points of these two materials. When water isemployed to effect the aforesaid separation, the overhead will be waterand the bottoms will be the azeotrope former.

Combinations of two or more of the above described processes may be usedto separate azeotrope former from the azeotropic distillate. Thus, anazeotrope comprising trioxane and nonaromatic hydrocarbons may be cooledto about F. at which temperature most of the trioxane crystallizes andmay be mechanically separated from the hydrocarbons contained in theazeotrope by mechanical means such as filtration. The separatedcrystalline trioxane may then be reheated and reused, as azeotropeformer Without further purification. The separated hydrocarbons may'thenbe washed with water to remove small amounts of trioxane which did notcrystallize and become separated in the above operation.

The preferred method with trioxane is to maintain the azeotropiccondensate at F. At this temperature two liquid phases are present, anupper phase of hydrocarbon oil containing very little trioxane and alower phase of trioxane substantially free of hydrocarbon oil. The upperphase is decanted and washed with water in the usual manner, the lowerphase may be recycled directly to the azeotroping column.

The azeotropic distillation may be effected atsired separation and themixture is passed through a heater and into a fractionating column wherethe mixture is subjected to fractionation. If desired, the azeotropeformer may be introduced directly into the fractionating column at apoint above the point of entry of thezhydrocarbon mixture and preferablyat a point near the top of the column and in this case the azeotropeformer acts in part as reflux for theiractionation. In the fractionatingcolumn the distillation is controlled so as to distill overhead lilcolumn which is provided with a heater or rean azeotrope comprising atleast one hydrocarbon component originally contained in the hydrocarbonmixture and azeotrope former leaving as undistilled residue at least onehydrocarbon component. By regulating the ratio of azeotrope former tohydrocarbon mixture entering the column, and by controlling thetemperature and pressure in the fractionating column the distillationcan be controlled so as to distill, for example, substantially all ofthe non-aromatic hydrocarbons contained in said hydrocarbon mixturetogether with substantially all of the azeotrope former, thereby leavingthe aromatic hydrocarbon originally present in said hydrocarbon mixtureas a residue substantially completely separated from non-aromatichydrocarbons and azeotrope former.

The aromatic hydrocarbon residue may be further purified and/or refinedby treatment with clay which may be accomplished at a temperature ofabout 230 F. employing l to 5 pounds of clay per barrel of the residue.In place of clay treatment or in addition to the clay treatment, the

aromatic residue may be treated with l to 10 pounds of sulfuric acid perbarrel of the residue followed by neutralization with caustic alkali orwith clay. If desired, the acid and/or clay treated stock may befractionally distilled to remove undesirable hydrocarbons and/orproducts of reaction.

The azeotropic distillate may be passed to a washing column providedwith packing material where the mixture enters. at a point near thebottom and is countercurrently washed with water which is led into thewasher at a point near the top of the column. The water dissolves theazeotrope former and the solution of azeotrope of the washer and passedthrough a heater and into a fractionating column maintained at such atemperature that the water vaporizes and distills leaving the azeotropeformer as a residue. This residue is recycled to the azeotropicdistillation column where it is again used as azeotrope former. Thewater distilled overhead is condensed and cooled and passed to thewasher column. As indicated hereinabove other solvents may be employedin place of water in this extraction process.

Other objects, features and advantages of my invention will be apparentto those skilled in the art from the following examples. However, itwill be observed that these examples are not to be taken as limiting myinvention since the process is applicable to separating other componentsfrom complex hydrocarbon mixtures employing the azeotrope formerdisclosed herein for efiecting the desired separation.

Example I former in water is withdrawn from the bottom A hydrocarbonfraction obtained by tracfraction having a boiling range of about 210 F.to about 235 F. and comprising substantially 42 parts by weight oftoluene, 9 parts by weight of olefins and 49 parts by weight of parafllnand naphthene hydrocarbons is mixed with trioxane boiler and a refluxcooling coil where the mixture is subjected to fractionation. Thedistillation is.

controlled so as to distill overhead an azeotrope consisting of theparaflin, naphthene and olefin hydrocarbons together with substantiallyall of the trioxane, leaving toluene as a residue, substantially.completely separated from non-aromatic hydrocarbons and trioxane. Thisis accomplished at a still head temperature of approximately 220 F. andat atmospheric pressure.

The overhead mixture or azeotrope is condensed and passed to a washingcolumn where it is washed countercurrently with water. The watersolution of trioxane is withdrawn from the bottom of the washer and isthus separated from the hydrocarbons present in the azeotrope.

The residue from .the azeotroping column amounting to about 40% byweight of the original hydrocarbon feed to the column has a gravity of31.2 A. P. I., a solubility in 99% sulfuric acid of 100% and an olefincontent of 0.1%.

Example II One part by weight'of a hydrocarbon fraction of hydroformedgasoline boiling between about 260 F. and 300 F. and comprising 76 partsby weight of xylene and 24 parts by weight of nonaromatic hydrocarbonsis mixed with 0.3 part by weight of trioxane and the resulting mixtureazeotropically distilled. At a distillation temperature of 230 F. atnormal atmospheric pressure, an azeotrope comprising substantially allof the non-aromatic hydrocarbons and all of the passed to a cooler wherethe temperature is reduced to about 100 F. At this temperature most ofthe trioxane is crystallized and removed by filtration. The crystallizedtrioxane is then melted and returned to .the azeotropingcolumn. Thehydrocarbon material from which the crystallized trioxane has beenremoved is passed into a water washer and contacted countercurrentlywith water to remove the remaining trioxane which is then separated fromthe aqueous solution in a conventional manner.

The foregoing description is not to be taken as in any way limiting butmerely as illustrative of my invention for many variations may be madeby those skilled in the art without departing from the spirit or thescope of the following claims:

I claim:

l. A process for the treatment of a complex hydrocarbon fraction toseparate one chemically similar component from the other chemicallysimilar components different from said first named chemically similarcomponents contained therein which ordinarily distill from saidhydrocarbon fraction in the same temperature range as said first namedchemically similar component distills therefrom which comprisesazeotropically distilling said complex hydrocarbon fraction in thepresence of a sufiicient amount of trioxane to vaporize at least one ofthe chemically similar components contained in said complex hydrocarbonfraction together with said trioxane thereby leaving at least one of thechemically similar components difierent from the vaporized chemicallysimilar components contained in said complex hydrocarbon fraction in theresidue.

2. A process for the treatment of a hydrocarbon fraction containingaromatic and nonaromatic hydrocarbons to separate said aromatichydrocarbons from said non-aromatic hydrocarbons contained therein whichordinarily distill from said hydrocarbon fraction in the sametemperaturerange as said aromatic hydrocarbons distill therefrom whichcomprises distilling said hydrocarbon fraction in the presence oftrioxane to vaporize said non-aromatic hydrocarbons together with saidtrioxane, thereby leaving aromatic hydrocarbons in the residuesubstantially completely separated from hydrocarbons other than aromatichydrocarbons.

3. A process for the treatment of a hydrocarbon fraction containingaromatic and nonaromatic hydrocarbons to separate said nonaromatichydrocarbons from said aromatic hydrocarbons which ordinarily distillfrom said hydrocarbon fraction in the same temperature range as saidnon-aromatic hydrocarbons distill therefrom which comprises distillingsaid hydrocarbon fraction containing aromatic andnon-aromatichydrocarbons in the presenceof a sufficient amount oftrioxane to vaporize said non-aromatic hydrocarbons together with saidtrioxane, thereby leaving said aromatic hydrocarbons in the residuesubstantially completely separated from non-aromatic hydrocarbons, saidhydrocarbon fraction containing aromatic and non-aromatic hydrocarbonshaving a boiling range of not more than 50 F. between the limits of 150F. to 330 F.

4. A process for the treatment of a hydrocarbon mixture comprisingtoluene and nonaromatic hydrocarbons boiling in substantially the sametemperature range as toluene which comprises distilling said hydrocarbonmixture in the presence of a suflicient amount of trioxane to vaporizesaid non-aromatic hydrocarbons together with said trioxane, therebyleaving toluene in the residue substantially completely separated fromnon-aromatic hydrocarbons.

5. A process for the treatment, of a hydrocarbon mixture comprisingxylene and non-aromatic hydrocarbons boiling in substantially the sametemperature range as xylene which comprises distilling said hydrocarbonmixture in the presence of a sufiicient amount of trioxane to vaporizesaid non-aromatic hydrocarbons together with said trioxane,thereby-leaving xylene in the residue substantially completely separatedfrom non-aromatic hydrocarbons.

6. A process as in claim 5 wherein said hydrocarbon mixture has aboiling range of approximately 260 F. to 300 F.

7. A process for the treatment of a hydrocarbon fraction containingnaphthene and paraffin hydrocarbons to separate the naphthenehydrocarbons from the paraflin hydrocarbons contained therein whichordinarily distill from said hydrocarbon fraction in the sametemperature range as the naphthene hydrocarbons distill therefrom whichcomprises distilling said hydrocarbon fraction containing naphthene andparafiin hydrocarbons in the presence of a sufficient amount of trioxaneto vaporize said paraflin hydrocarbons together with said trioxanethereby leaving naphthene hydrocarbons in the residue substantiallycompletely separated from parafiln hydrocarbons.

GEORGE R. LAKE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,162,963 McKittrick June 30,1939 2,313,537 Greenburg Mar. 9, 1943 2,360,655 Deanesly Oct. 17, 19442,265,220 Sullivan Dec. 9, 1941 2,332,370 Cole Oct. 19, 1943 2,304,080Frank Dec. 8,, 1942 2,270,135 Mikesky et a1 Jan. 13, 1942 2,347,447Walker Apr. 25, 1944 2,397,839 Clark Apr. 2, 1946 FOREIGN PATENTS NumberCountry Date 831,295 France May 30, 1938 OTHER REFERENCES Rossini,Proceedings 21st Annual Meeting. American Petroleum Institute, Sec. 11,Refinery, Chicago, 111., Nov. 11--15, 1940, pages 43-47. Copy in Div.25.

